CN101975914A - On-line monitoring method and device for insulating state of power cable - Google Patents

Abstract

The invention discloses an online monitoring method and device for the insulation state of a power cable. The monitoring of the cable insulation state mainly considers two factors, one is the low-frequency current value generated by the low-frequency oscillation of the power system, and the other is the low-frequency current value generated by calculating the oscillation of the power system. The phase angle difference between the generated low-frequency voltage signal and the current signal, and then calculate the tangent value to obtain the cable dielectric loss factor value. When any one of the two factors is greater than the corresponding insulation safety threshold, it indicates that there is a hidden danger of insulation aging in the cable. The invention adopts system low-frequency signals for monitoring, has high detection accuracy, can clearly reflect the cable aging state, and does not need additional superimposed power supply, thereby avoiding harm to the safety of the power grid.

Description

A method and device for on-line monitoring of power cable insulation status

technical field

The invention relates to the field of on-line monitoring of power equipment, in particular to an on-line monitoring device for the insulation state of a power cable.

Background technique

There are offline detection method and online monitoring method for cable insulation monitoring. The test of offline detection method needs to be carried out with power failure. After the power failure, the status of the cable is inconsistent with that in operation, which affects the accuracy of judgment; the offline detection method is a periodic inspection, not continuous monitoring at any time. It has greater blindness, resulting in greater waste of manpower and material resources, and even unnecessary insulation damage. For example, in the insulation preventive test, since the AC polyethylene cable insulation cannot use the DC withstand voltage test, the AC withstand voltage test or the resonance withstand voltage test must be used, but these equipment are huge, and the test is time-consuming and laborious. Moreover, it is found through research that The withstand voltage test not only has limited ability to detect defects, but also has great damage to the cable insulation. The online monitoring method enables the operator to grasp the real-time information of the cable operation at any time without power off, helping to judge whether the insulation of the cable has dropped and whether maintenance is required, thus reducing power outage time, reducing equipment damage caused by maintenance and improper maintenance. The advantages of reducing accidents, reducing manpower, material resources, and financial resources required for maintenance, and reducing on-duty personnel and man-made accidents are the current development trends in insulation monitoring of power equipment.

At present, there are many online test methods proposed at home and abroad that can be used for XLPE high-voltage cables, such as DC superposition method, DC component method, loss factor method, partial discharge method, dielectric loss detection method, etc. These methods are theoretically feasible. , but there are still some technical defects, so there is no report on the field application of intelligent online monitoring. From the specific analysis of these technologies, the DC component method is suitable for the on-line monitoring of cable insulation systems of various voltage levels, but due to the interference of stray currents and the difficulty of extracting small currents, it is not suitable for practical applications in power systems; The superposition method is suitable for the monitoring of the low-voltage cable insulation system, but it is not suitable for the practical application of the high-voltage power system because it cannot solve the contradiction between the grounding method and the DC signal loading; the partial discharge method is an effective method for judging the cable insulation aging, but it cannot When solving the interference of the power system, the partial discharge test of the power cable can only be used as a means of quality assessment of the cable products before leaving the factory and is limited to a well-shielded laboratory, which cannot be used for on-site testing.

Contents of the invention

In view of the defects and deficiencies in the above technologies, the purpose of the present invention is to provide an on-line monitoring device for the insulation state of power cables, which uses system low-frequency signals for monitoring, has high detection accuracy, can clearly reflect the aging state of cables, and does not require additional superimposed power supplies , to avoid harm to the grid security.

An on-line monitoring method for the insulation state of a power cable, specifically: collecting the current of the cable ground wire and the voltage signal on the main insulation of the cable, obtaining the low-frequency current and voltage signal oscillating in the power system through filtering, and calculating the phase angle difference between the two, Calculate the tangent value of the phase angle difference to obtain the cable dielectric loss factor value. When the cable dielectric loss factor value is greater than the dielectric loss safety threshold, it indicates that the cable has a hidden danger of insulation aging.

An on-line monitoring method for the insulation state of a power cable, specifically: collecting the current of the grounding wire of the cable, and obtaining the low-frequency current oscillating in the power system through filtering, and if the low-frequency current is greater than the current safety threshold, it indicates that the cable has a hidden danger of insulation aging.

The frequency of the low-frequency current signal is 0.1-2.5 Hz.

An online monitoring device for the insulation state of a power cable, including a current transformer, a voltage transformer, a filter amplifier circuit, an A/D conversion module, a data transmission module, and a monitoring module. The current and voltage signals are sent to the filter amplifier circuit, and the filter amplifier circuit filters out the low-frequency current and voltage signals oscillating in the power system and amplifies them to the A/D converter. The A/D converter converts the low-frequency current and The voltage signal is transmitted to the monitoring module through the data transmission module. The monitoring module calculates the phase angle difference between the low-frequency current and the voltage signal, and obtains the tangent value of the phase angle difference to obtain the cable dielectric loss factor value. When the cable dielectric loss factor value is greater than the dielectric loss safety Threshold, start the alarm.

An on-line monitoring device for the insulation state of a power cable, comprising a current transformer, a filter amplifier circuit, an A/D conversion module, a data transmission module and a monitoring module, the current transformer transmits the collected current signal of the cable ground wire to the filter amplifier circuit, The filter amplifier circuit filters out the low-frequency current signal oscillating in the power system and amplifies it and sends it to the A/D converter. If the low-frequency current is greater than the current safety threshold, an alarm is activated.

The technical effect of the present invention is embodied in that the source of the low-frequency signal in the present invention is the low-frequency signal generated by the oscillation of the power system. When the power systems are interconnected through tie lines, under the action of small disturbances, the generator rotors will sway relative to each other, making the active power on the tie lines fluctuate within a certain range at a very low frequency (0.1-2.5Hz) , called low-frequency oscillation. With the increasing scale of the interconnected power system, the low-frequency oscillation problem caused by the interconnection of the system has become one of the most important factors that endanger the safe operation of the power grid and restrict the transmission capacity of the power grid. This problem has not been completely solved yet, but it provides a natural low-frequency signal for cable insulation monitoring. This unfavorable phenomenon can be reasonably used to avoid artificially inputting low-frequency signals to the power grid system again, which will affect the operation safety of the power system.

Description of drawings

Fig. 1 is a schematic diagram of the structure of the device of the present invention.

Fig. 2 is a schematic view of the hardware protection part of the present invention.

Figure 3 is a structural diagram of the filter amplifier circuit.

Figure 4 is a flow chart of insulation state analysis.

Detailed ways

In the present invention, the current sensor is set on the grounding wire of the cable, and the current transformer converts the current signal into a voltage signal by connecting a small resistance, and is connected to a filter amplifier circuit. The circuit filters out the required low-frequency current and voltage signals, amplifies them by a certain factor, and then sends them to the A/D converter. The microcomputer control module then sends the current and voltage digital signals obtained from the A/D converter to the GSM/GPRS data transmission module ( DTU), GSM/GPRS DTU sends the current signal, voltage signal and line number to the GSM/GPRS DTU in the monitoring room, and the GSM/GPRS DTU in the monitoring room finally transmits the signal to the monitoring host through the serial port. After the monitoring host receives the current and line number information of each cable, it first stores it in the database and displays it, and then analyzes and judges the information, which can automatically generate the trend chart of current and dielectric loss factor, and through the expert intelligent diagnosis system Carry out fault diagnosis and alarm. The principle and specific implementation of the present invention will be described in detail below.

1. Principles and methods

One of the principles of the present invention is that the dielectric loss factor δ has the following relationship with the applied voltage angular frequency ω, sample capacitance C, and sample equivalent resistance value R: tanδ=1/(ωRC). Due to the improvement of manufacturing technology and the progress of processing technology, the power frequency tanδ of degraded insulation is very small, so the measurement of dielectric loss factor of insulation has strict requirements on the measurement equipment. It can be seen from the above formula that the dielectric loss value is approximately inversely proportional to the angular frequency ω of the applied test voltage. Therefore, reducing the frequency of the applied test voltage can significantly improve the accuracy of on-site detection of insulation tanδ.

The second principle on which the present invention is based is that the change of the low-frequency parameter of the cable is more sensitive to the aging degree of the cable.

The third principle on which the present invention is based is that the cable grounding full current contains abundant information on the deterioration of the main cable insulation. When the main insulation of the cable is aging, there will be changes in the low-frequency current amplitude and phase in the leakage current on the grounding wire of the cable, and the degree of this change can be used as the basis for judging the aging degree of the cable.

2. Detailed description of the hardware, see Figure 1

1) Protective measures

Multiple protections are carried out through fast fuses and ceramic discharge tubes. The advantages and disadvantages of these protections complement each other, which can play a very good protective role and ensure the safety of personnel and equipment. See Figure 2

(1) Fast fuse: The fast fuse is used to ensure the absolute safety of people and equipment. In case of insulation breakdown of the cable, the fuse will be broken quickly and the test circuit will be automatically disconnected.

(2) Ceramic discharge tube: the present invention adopts the ceramic discharge tube of model B8G070H, which is a mature product mass-produced by a certain factory. When the power frequency threshold voltage is less than 70V), discharge occurs. Under normal circumstances, the equivalent resistance of the discharge tube is close to infinity, which protects the safety of measuring instruments and operators.

2) Power module

The power module provides power for the lower computer to work. After choosing a solar panel of a suitable size and a storage battery of a suitable capacity, this solution does not require complex wiring and frequent replacement of batteries, and can provide continuous power to the lower computer.

3) Filter amplifier module

The filtering and amplifying module is mainly used to filter out useful low-frequency signals and amplify them to V level, and effectively attenuate the interference signals. See Figure 3. The main interference signal of the present invention is the 50Hz power frequency signal and high-order harmonics at all levels. The filter circuit is mainly composed of two second-order Butterworth low-pass filters to attenuate the 50Hz power frequency signal to the mV level, and the signal passes through the filter circuit Finally, only the required low frequency signal (μV level) and power frequency interference signal (mV level). The 1Hz signal is amplified to V level by the amplifying circuit for various processing, and the interference signal is attenuated to μV level. The magnification factor of the amplifier is 100000. For 1nA current input, the voltage is 1V, the amplified signal is 0.1V, and the display is 1nA; 50nA is 5V, and the display is 50nA; when it is greater than 50nA, the range is automatically changed, and the actual value is multiplied by 20. The amplifier attenuates the 50Hz power frequency signal by more than 80dB.

This circuit is linear in the range of input signal 1nA～1000nA, and the error is 5%. The amplifying circuit greater than 1000nA tends to be saturated gradually, and the linearity becomes worse. Due to the large current, it can be considered that the cable is aging and needs to be repaired or replaced. If it is less than 1nA, the influence of the offset, bias and thermal noise of the components will increase, which will affect the linearity, but such a weak current indicates that the cable insulation is good, and the inaccurate value will not affect the judgment. In general, different cables have different aging current criteria, and the magnification can be appropriately adjusted according to the actual cable to meet different measurement ranges.

4) Microcomputer control module

The main function of the microcomputer control module is to collect the signal processed by the amplifier circuit, and perform unified phase calculation on it to realize the control of the filter amplifier module, signal acquisition module, and GSM/GPRS communication module, which is the key link to realize system automation.

5) GSM/GPRS communication module

GSM/GPRS DTU is a data transmission and remote monitoring terminal equipment based on GSM/GPRS communication network. This module is based on the latest GSM/GPRS digital mobile communication network, which overcomes the shortcomings of short communication distance and unstable performance. Main function: complete the data communication between the upper and lower computers.

6) GPRS networking mode

Considering that substations generally cannot be connected to the Internet, and it is expensive to apply for an APN private network from a mobile company, there is no prospect of large-scale application in the power system. The present invention adopts central primary and secondary GSM/GPRS DTU, mobile intranet dynamic IP + mobile DNS analysis service. In this solution, the customer first contacts the mobile DNS service provider to activate the mobile dynamic domain name, and the monitoring point first uses the domain name addressing method to connect to the mobile DNS server, and then the mobile DNS server finds the central mobile dynamic IP and establishes a connection. The center also uses GSM/GPRS DTU as the receiving end, but the GSM/GPRS wireless center is not as stable as the wired one, so two primary and secondary GSM/GPRS DTUs are used for redundant backup. When the GSM/GPRS DTU receiving end of the main center is offline, all monitoring points are automatically transferred to the GSM/GPRS DTU receiving end of the sub-center. This method can also greatly save the cost of fixed IP.

7) Current and voltage transformers

Both the current transformer and the voltage transformer are high-precision Rogowski coil current transformers, and the magnetic core is a ring-shaped film alloy core.

3. Software

The software part includes serial port communication module, DTU setting module, database, control and alarm module, drawing and data display module.

1) Serial communication module

The serial receiving module mainly realizes data receiving. After all the data is received into the cache and stored in the database at one time, a series of transformations are performed on the effective data to finally convert it into the real current value collected, and then stored in the array. Waiting for data preprocessing.

2) database

The present invention intends to adopt a small database Access, which can conveniently operate the database.

3) Control and alarm module

The control module is mainly composed of data validity judgment, data clearing and saving, alarm and other parts. The control part is complicated but very important, and the control flag is mainly realized by some Boolean data. The monitoring of the insulation state mainly considers two factors, one is the collected low-frequency current value, and the other is the cable dielectric loss factor obtained by calculating the phase angle difference between the low-frequency voltage signal and the current signal, and then calculating the tangent value. When any one of the two factors is greater than the corresponding insulation safety threshold, the system will automatically alarm. The dielectric loss safety threshold corresponding to the cable dielectric loss factor value is an empirical value confirmed by this method when the cable safety state is known. The current safety threshold corresponding to the low-frequency current is determined in the same way as the dielectric loss safety threshold. Figure 4 shows the flow chart of the insulation state analysis of the control module.

4) DTU setting module

This module is mainly used to set up GSM/GPRS DTU. Including communication port settings, DTU polling time settings, etc.

5) Drawing module

According to the collected current value, the current curve and the insulation loss factor curve are drawn in real time through the drawing module, so as to effectively monitor the amplitude of the current signal in real time and the online monitoring of the cable insulation loss factor. The current and insulation loss factor curves drawn by the drawing module are displayed through the graphic display module, so as to realize online real-time monitoring of high-voltage cable insulation.

Claims (6)

1. A method for on-line monitoring of the insulation state of a power cable, specifically: collecting the current of the cable ground wire and the voltage signal on the main insulation of the cable, obtaining the low-frequency current and voltage signals generated by the oscillation of the power system through filtering, and calculating the low-frequency current and voltage The phase angle difference between the signals is obtained by calculating the tangent value of the phase angle difference to obtain the cable dielectric loss factor value. When the cable dielectric loss factor value is greater than the dielectric loss safety threshold, it indicates that the cable has a hidden danger of insulation aging. 2. The method for on-line monitoring of power cable insulation status according to claim 1, characterized in that the frequency of the low-frequency current and voltage signals is 0.1-2.5 Hz. 3. An online monitoring method for the insulation state of a power cable, specifically: collecting the current of the cable grounding wire, and obtaining the low-frequency current oscillating in the power system through filtering. If the low-frequency current is greater than the current safety threshold, it indicates that there is a hidden danger of insulation aging in the cable. 4. The method for on-line monitoring of the insulation state of a power cable according to claim 3, wherein the frequency of the low-frequency current signal is 0.1-2.5 Hz. 5. An on-line monitoring device for the insulation state of a power cable according to claim 1, comprising a current transformer, a voltage transformer, a filter amplifier circuit, an A/D conversion module, a data transmission module and a monitoring module, a current transformer and a voltage transformer The transformer transmits the collected current of the cable grounding wire and the voltage signal on the main insulation of the cable to the filter amplifier circuit, and the filter amplifier circuit filters out the low-frequency current and voltage signals oscillating in the power system and amplifies them and sends them to the A/D converter. The D converter transmits the low-frequency current and voltage signals obtained after analog-to-digital conversion to the monitoring module through the data transmission module. The monitoring module calculates the phase angle difference between the low-frequency current and voltage signals, and obtains the tangent value of the phase angle difference to obtain the cable medium Loss factor value, when the cable dielectric loss factor value is greater than the dielectric loss safety threshold, an alarm is activated. 6. An on-line monitoring device for the insulation state of a power cable according to claim 2, comprising a current transformer, a filter amplifier circuit, an A/D conversion module, a data transmission module and a monitoring module, the current transformer will collect the cable grounding wire The current signal is transmitted to the filter amplifier circuit, and the filter amplifier circuit filters out the low-frequency current signal oscillating in the power system and amplifies it to the A/D converter. The A/D converter transmits the low-frequency current signal obtained after analog-to-digital conversion through data transmission The module transmits to the monitoring module, and the monitoring module judges that if the low-frequency current is greater than the current safety threshold, an alarm is activated.

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